Digital printers, such as inkjet, laser printers, dot matrix printers, LED printers, or gel printers, (to name a few) have been around for many years. They can be black and white or colour printers (e.g. four colour, or six colour, or more). They print onto a print medium, typically paper, cardboard, plastics, film or the like. Paper comes in different kinds with different colours and different ink-retention properties.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a print medium in a particular order to form alphanumeric characters, area-fills, and other patterns. Low cost and high quality of the hardcopy output, combined with relatively noise-free operation, has made inkjet printers a popular alternative to other types of printers used with computers.
Inkjet printing involves the ejection of fine droplets of ink onto a print medium such as paper, transparency film, or textiles in response to electrical signals generated by a microprocessor. There are two basic means currently available for achieving ink droplet ejection in inkjet printing: thermally and piezoelectrically. In piezoelectric inkjet printing, the ink droplets are ejected due to the vibrations of piezoelectric crystals, again in response to electrical signals generated by the microprocessor.
In thermal inkjet printing, an inkjet image is formed when a precise pattern of dots is ejected from a drop generating device known as a “print head” onto a printing medium. The typical inkjet print head has an array of precisely formed nozzles (or ejector portions) attached to a thermal inkjet print head substrate, such as silicon, nickel, or polyamide, or a combination thereof. The substrate incorporates an array of firing chambers or drop ejector portions that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each firing has a film resistor, known as a “firing resistor” located opposite the nozzle so ink can collect between the firing resistor and the nozzle. The print head is mounted on a carriage that travels along the width of the printer (otherwise referred to as the “scan axis”).
Digital printers have a number of “print modes” configured in them at the source of manufacture to set parameters and variables to ensure that a satisfactory print job is achieved. This can include ensuring that the correct ink load is applied when printing on a specific media, and that a required Image Quality (IQ) is produced. Other parameters that are defined by the print mode can include the number of passes of a printer head over a print medium, the dots per inch (dpi) that defines the number of dots of ink that are printed onto a page, or values in a Look-Up-Table (LUT) that define properties of ink composition for the printed page, for example how different coloured inks are mixed together to produce a desired colour.
Further parameters include the carriage speed and firing frequency of the print head. The carriage speed is measured in inches per second (ips), at which the printer carriage (and the print heads in it) move across the media. Typical carriage speeds range from 20 to 60 ips. The firing frequency is the number of drops fired per second. The higher the firing frequency, the higher the dot placement error of the drops on the media. High dot placement errors provoke undesired IQ defects (grain in area fills, blurred lines, etc.). Firing frequency is a parameter that is closely linked to the number of passes of the print mode (the higher the number of passes, the lower the firing frequency) and to the carriage speed (the higher the carriage speed the higher firing frequency of the pen).
The number of passes of the printer head over a print medium defines the number of times that the printer head passes over the same region of the document whilst applying ink to the medium. Each time a subsequent pass of the printer head is made, ink is applied on top of ink that has been applied in an earlier pass of the printer head.
Each print mode will have a different combination of values for the parameters that control aspects of the print operation. For example, one set of print modes may have constant values for all parameters except the number of passes, where each print mode defines a different number of passes of the print head that are performed during a print operation. There may also be other sets of print modes where the number of passes is kept constant and another parameter, for example dpi, is set at different values in each of the print modes within the set. Having a number of different print modes enables a user to select a print mode that strikes a balance between throughput of print jobs and image quality of the print job.
The wrong selection of the print mode can be two-fold. On one hand the user may select higher image quality print modes than needed. This causes a waste of time due to the slow throughput of print jobs associated with these print modes. This is because the printer takes a longer time to perform a print operation with a higher image quality print mode than a lower image quality print mode, as more ink is applied, and/or more passes of the printer head are typically performed, when performing a print operation with a higher image quality print mode.
On the other hand, if the user selects a lower image quality print mode than required, it is likely that the final printout will not meet the image quality expectations, especially due to a defect known as “banding” as will be discussed in more detail later.
In some cases a printed medium may have to be scrapped (thrown away) if it does not have a high enough image quality and the user will be forced to repeat the print job with an associated waste of money and time. Selection of the correct print mode helps to achieve optimal, or better, performance.
Some digital printers accommodate different kinds of paper/media by having a plurality of different print modes, different print modes being associated with specific types of media. Some printers can also have print modes associated with more than one print quality for each supported media, for example greyscale, normal and photo quality. For example, if there are three types of media supported and three print qualities available, the printer will have nine different print modes.
The user can select an appropriate print mode to be used for printing a particular print job with a particular printer from a group or set of pre-set possibilities in accordance with which media will be used and which print quality is required.
Typically users lack the knowledge and understanding to correctly select the optimal print mode for their particular needs. Moreover, the media portfolio can be very large and often confusing. Sometimes users choose not to spend the time necessary to select the correct print mode. This is a recognised usability issue.
A user may select the print mode that provides the best possible image quality in order to avoid the risk that an unacceptable image quality will be produced. Selecting an unnecessarily high print mode is inefficient and reduces the throughput of print jobs that could be achieved if the correct print mode were selected. Some print jobs can be printed faster, still with acceptable results, with a lower image quality print mode. Selecting an unnecessarily high image quality print mode further wastes ink and printing time, as ink is printed onto the medium that does not substantially improve the image quality, or improves the image quality to an unnecessarily high level.
Alternatively, a user may trial and error with different print modes to determine the most efficient print mode that provides them with an acceptable image quality. This also wastes time and ink during the trials, and also paper with associated cost and refuse disposal issues.
An unacceptable image quality can include an effect known as “banding”. Banding occurs where an insufficient amount of ink has been printed onto the medium, and lighter colour bands are created across the image where there is a shortage of ink. Banding shows up as straight bands or lines that follow the direction of the printer carriage motion. Alternatively, banding may be caused where ink is printed onto incorrect regions of the medium, and some areas do not have the ink that they should have.
There are a number of causes of banding, and the most common cause of banding is where an incorrect print mode has been selected. The incorrect print mode causes too little ink to be printed onto the medium, or causes the ink to be printed onto the medium in too few passes of the print head over the medium (for example, in an inkjet printer, such as a thermal or piezoelectric inkjet printer).
It is easy to see banding in low pass print modes. High pass print modes can be very robust to this type of image quality defect.
Banding can also occur where a nozzle in a print head is blocked, where a nozzle is not aligned correctly and therefore does not fire the ink onto the correct region of the media, or the nozzle is otherwise not firing correctly. Whether or not banding occurs is also influenced by the accuracy/quality of the print medium, as this determines how much ink is required for the print medium to provide an acceptable image quality. For example, the porosity of the print medium, matte or glossy finish, or colour of the print medium can alter the desired print mode.
Although the invention is particularly concerned with inkjet printing, the invention may also find application with regard to other printing technologies, such as laser printing, LCD printing, LED printing, LEP gel printing, and/or toner-based printing (e.g. photocopiers), and other printers. The list above is not exhaustive.
According to a first aspect of the invention, there is provided a method of printing a document (100; 400) using one of a plurality of print modes, the method comprising:                (i) notionally dividing a digital image of the document (100; 400) into sectors (102, 202, 302; 402, 502);        (ii) for each sector (102, 202, 302; 402, 502), comparing one or more properties of the luminosity of pixels within each sector with one or more threshold values;        (iii) using the comparison of (ii) to determine whether each sector (102, 202, 302; 402, 502) is likely to show an image defect if printed at a lower image quality print mode;        (iv) selecting a print mode such that all sectors (102, 202, 302; 402, 502) are determined not to be likely to show an image defect;        (v) printing the document (100; 400) using the selected print mode.        
Comparing a property of the luminosity of pixels within each sector may involve determining the mean value and/or standard deviation of the luminosity of the pixels and comparing it/them with one or more threshold values.
Thus a document can now be printed with an optimal print mode that has been determined dynamically and automatically, without a user's input, using information obtained from the document that is to be printed. This can avoid a user accidentally (or deliberately) selecting an inappropriate print mode for printing the document. Also, it is not necessary for a user to select any print mode: the printer/processor automatically selects a suitable print mode.
Another aspect of the invention relates to a printed document produced using the method of the first aspect of the invention.
According to another aspect of the invention, there is provided a printer system comprising a printer, computer memory having a plurality of print modes stored thereon, and printer driver software;                the system being configured such that the printer driver software is adapted to compare one or more properties of the luminosity of pixels within a document that is to be printed with one or more threshold values, and to select one of the plurality of print modes that is to be used to print the document based on the comparison.        
According to a further aspect of the invention, there is provided printer control software which is adapted, when loaded on a processor associated with a printer, to compare at least one property of the luminosity of pixels in a document that is to be printed with at least one threshold value, and to select one of a plurality of print modes that is to be used when printing the document based on the comparison.
The software may be stored on a processor that is located on the printer, or may be located in an off-printer processor that is in communication with the printer.
According to another aspect of the invention, there is provided a printer system comprising means for printing, and means for data processing, the system being such that, in use, the means for processing compares a property of the luminosity of pixels within a document that is to be printed with a threshold value and determines an optimal print mode associated with the document, and the means for printing prints the document using the determined print mode.
According to a further aspect of the invention there is provided a method of calibrating a printer comprising analysing properties of a document that is to be printed, and calibrating printer settings based on the analysis.
According to a further aspect of the invention there is provided a method of printing a document using one of a plurality of print modes, the method comprising:                analysing a property of the luminosity of pixels within the document;        selecting an optimal print mode for printing the document from one of the plurality of print modes based on the analysis;        printing the document using the determined print mode.        
According to a further aspect of the invention there is provided a method of reducing the likelihood of image quality defects in a printed document comprising:                notionally dividing the document into a grid of sectors;        comparing at least one property of the luminosity of pixels within each sector with at least one threshold value;        associating an individual optimal print mode associated with each sector based on the comparison for that sector;        determining an optimal print mode for the document as a whole based on the individual optimal print modes associated with each sector; and        printing the document using the optimal print mode; wherein the optimal print mode provides a print job with reduced image quality defects when compared with a print job provided by other print modes.        
According to a further aspect of the invention there is provided a method of selecting a print mode for printing a document comprising:                analysing at least one property of the luminosity of pixels within the document;        comparing the analysis of the at least one property of the luminosity of the pixels with at least one threshold value; and        selecting a print mode for printing the document based on the result of the comparison.        
According to a further embodiment of the invention, there is provided a method of printing a document (100; 400) using one of a plurality of print modes, the method comprising:                notionally dividing the document (100; 400) into sectors (102, 202, 302; 402, 502);        comparing one or more physical characteristics of pixels within each sector as they will appear in the printed document, with one or more threshold values;        determining one of the plurality of print modes as being the minimum print quality suitable for printing that sector based on the comparison of the physical characteristics of pixels for that sector;        determining the optimal print mode for the document based on the minimum print modes determined to be suitable for each sector; and        printing the document using the determined optimal print mode.        